**Core Concept:** Anembryonic pregnancy, also known as a "blighted ovum," is a type of ectopic pregnancy where a fertilized egg implants in the uterus but does not develop into a fetus. The non-visibility of the embryo within the gestational sac is a key feature.

**Why the Correct Answer is Right:** In a normal pregnancy, the embryo develops and its size starts to increase as the gestational sac expands to accommodate the growing fetus. However, in an anembryonic pregnancy, the embryo does not develop, leading to a smaller gestational sac diameter. As a result, the absence or small size of the embryo within the gestational sac is a strong indicator of anembryonic pregnancy.

**Why Each Wrong Option is Incorrect:**
A. A gestational sac diameter alone is not sufficient to diagnose anembryonic pregnancy. The presence of an embryo within the gestational sac is essential for a definitive diagnosis.
B. Anembryonic pregnancy is characterized by a small gestational sac diameter due to the lack of an embryo.
C. Lack of an embryo within the gestational sac is a defining feature of anembryonic pregnancy, regardless of gestational sac diameter.
D. Anembryonic pregnancy is characterized by a small gestational sac diameter due to the absence of an embryo.

**Clinical Pearl:** In cases where the embryo is not visible within the gestational sac, evaluating the gestational sac diameter can help narrow down the differential diagnosis. However, a definitive diagnosis of anembryonic pregnancy requires both the absence of an embryo and a small gestational sac diameter.

**Correct Answer:** D. A gestational sac diameter of <20 mm is suggestive of anembryonic pregnancy, if the embryo is not visible.